Expectant mothers who take aspirin may produce sons with unusually low libidos, a rat study suggests. It is not known whether a similar effect occurs in humans, but the research reinforces the need for prudence when taking any medication during pregnancy.

Pregnant rats supped on water laced with soluble aspirin for two weeks around the time they gave birth. But as their sons matured, their sex drives didn't.

The males were slower at initiating sex than normal littermates, taking two to three times longer to mount females. They were also less likely to penetrate females and less likely to ejaculate, the researchers report in Nature Neuroscience1.

Mothers took a dose each day that was equivalent to the dose a human would take for a headache. Even so, "I don't think there's any need to panic," says psychologist Marc Breedlove from Michigan State University, who was not involved in the study. "It is a subtle effect even in rats, but it is a reminder for pregnant women to use caution."

Pill poppers

Aspirin lowers levels of prostaglandins, a group of chemicals known to affect cell signalling and inflammation. This is the first time that prostaglandins have been implicated in brain development, says the study's co-author, Margaret McCarthy from the University of Maryland, Baltimore.

But several other over-the-counter medicines also affect prostaglandins. Millions of people regularly take drugs to counter stomach ulcers, pain and arthritis, and many of these lower prostaglandin levels.

"There are many different prostaglandins, doing a lot of things that we don't understand," says McCarthy. The new result highlights the need to develop drugs that target specific prostaglandins, so unwanted side-effects can be avoided, she says.

Gender bender

The study sheds light on how brains become either male or female. Early in development, the brains of both sexes are identical. But later on, differences emerge. For example, a region called the sexually dimorphic nucleus is up to seven times larger in male brains than in female.

The brain of a fetus becomes male as the gonads pump out male sex hormones, flooding the brain with testosterone. In turn this is converted to estrogen, which leads to the development of male characteristics. But just how estrogen triggers masculinization is not known.

Because drugs that lower prostaglandin levels inhibit the development of male sexual behaviour, McCarthy believes that prostaglandins must be involved in telling the brain to turn into one sex or the other.

Her team carried out a second experiment that supports this theory. The team wanted to know how increased prostaglandin levels would affect female development, so they injected newborn female rat pups with a type of prostaglandin called PGE2. When the animals grew up their sexual behaviour was similar to that of males, as they mounted other females and pretended to mate with them.

Their brain structure was also subtly different. PGE2-treated female rats had more nerve communication terminals, known as synapses, in the brain area related to sexual function. So the researchers think that prostaglandins affect behaviour by altering the brain's wiring.

Drugs that raise prostaglandin levels are sometimes given to infants with a particular type of heart defect. Care should be taken to make sure these drugs do not have any untoward effects, McCarthy warns.